Adding a 'SUN' via lamp_add() actually adds a 'HEMI'. Bug?
Update method on renderman.renderman_type

Setting renderman_type to SKY sets the lamp type to 'HEMI' See renderman code

    if light_type in ['SKY', 'ENV']:
        lamp.type = 'HEMI'


This is not a bug.  bpy.ops.object.lamp_add(type='SUN') adds a lamp object of type 'SUN' named "Sun",  which is pretty easy to test.